CD4 has been proposed to aid co-localization of the T cell receptor (TCR) with MHC class II molecules, and to transduce signals into the T cell via the tyrosine kinase, p56 . Certain TCRs appear to recognize flanking residues on MHC class II-bound peptides. However, due to the random nature of antigen processing, not all peptides possess these flanking residues and the intimate association between CD4 and the TCR is required to ensure maximal stimulation in their absence. Further studies have shown that an epitope in the D3 domain of CD4 is critical in mediating these events. The long term aim of this proposal is to determine the physiological significance of CD4:TCR interaction, and the influence of MHC class II bound peptide flanking residues on T cell function. The first specific aim will use site directed mutagenesis to further define the exact amino acid residues involved in this interaction on both CD4 and the TCR:CD3 complex. This information will be used to produce a mouse model in which the wild- type CD4 gene will be replaced by homologous substitution with a point mutant functionally incapable of interacting with the TCR. The physiological significance of this interaction can then be assessed in vivo. The second aim of this proposal will determine whether the variability of peptide flanking residues generated during antigen processing can affect T cell function. Studies will be conducted to determine if there is a correlation between the residues recognized by the TCR and CD4 "dependence." Given the ramifications of inappropriate T cell activation on peripheral effector functions, the dissection of the critical regulatory mechanisms which control these events is likely to lead to many novel therapies for autoimmune conditions and new strategies for improving vaccination protocols. It is hypothesized that the requirement for CD4-CD3:TCR interaction has evolved to compensate for the inability of MHC class II- restricted TCR to recognize the myriad of naturally processed peptides that derive from a single antigenic epitope.